The two incoming transmission lines of a telephone instrument are typically connected to the terminals of a hookswitch, the hookswitch being a pair of switches operated by lifting the handset of the instrument. With the handset down, or "on-hook", the hookswitch contacts connects the two incoming lines to a ringing circuit, isolating the speech circuitry from the lines, whereas with the handset up, or "off hook" they connect the incoming lines to a rectifier supplying the speech circuitry of the instrument. The rectifier provides the unidirectional voltage of the correct polarity required by the electronic speech and operating circuitry of the instrument, the polarity of the voltage across the pair of incoming lines being unknown.
In this way, the hookswitch isolates the speech and operating circuitry from the incoming lines until the subscriber goes "off-hook".
The hookswitch contacts are subject to a quiescent line voltage of approximately -50 volts, a ringing voltage of approximately 80 volts RMS, and, in exceptional circumstances such as lightning strikes and line testing, voltage surges of a maximum of 240 volts. The manufacturing costs of a hookswitch which can tolerate these voltages are relatively high in comparison with other parts of the telephone instrument, particularly because of the assembly of mechanical parts that is required.